But she has other concerns, particularly about her 7- and 12-year-old daughters. The long hospitalization for gene therapy followed by months of recuperation would be hard on them, she said.

“They are not babies, but they need me,” Ms. Polanco said.

Patients ask when in the course of the disease is gene therapy needed. If they wait too long, the disease can lead to a stroke or permanent to damage organs and bones. But if the disease is mild and manageable, is it better to wait? André Marcel Harris, 33, who lives in Houston, decided that waiting is his best option.

“I am not as sick as many others,” he said. “At this point, gene therapy is not part of the conversation.”

Shamar Lewis, 20, who lives in Orlando, Fla., has several concerns — possible long term side effects of gene therapy, costs and the monthslong process of being treated and recovering.

Still, sickle cell has been such a burden. He spent years of early adolescence unable to go to school because he was so sick — his school even sent teachers to his home. He has now finished high school and is interested in joining the military but has since learned that, with sickle cell, he would not be accepted. With gene therapy, “I would be able to truly live my life,” he told his mother, Carla Lewis.

But what would that life be like?

Without the disease, said Teonna Woolford, 31, a sickle cell patient and advocate who lives in Owings Mills, Md., patients “don’t know who they are.” Many worry, she said, about “trading one familiar issue — sickle cell — with something else and overall fear of the unknown.”